feat(02-02): implement ELK diagram renderer with SVG/JSON content negotiation

- ElkDiagramRenderer: ELK layered layout (top-to-bottom) with JFreeSVG rendering
- Color-coded nodes: blue endpoints, green processors, red errors, purple EIPs, cyan cross-route
- Compound node support for CHOICE/SPLIT/TRY_CATCH swimlane groups
- DiagramRenderController: GET /api/v1/diagrams/{hash}/render with Accept header negotiation
- DiagramBeanConfig for Spring wiring
- 11 unit tests (layout, SVG structure, colors, compound nodes)
- 4 integration tests (SVG, JSON, 404, default format)
- Added xtext xbase lib dependency for ELK compatibility

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

cameleer3-server-app/pom.xml

@@ -61,6 +61,11 @@
             <artifactId>org.jfree.svg</artifactId>
             <version>5.0.7</version>
         </dependency>
+        <dependency>
+            <groupId>org.eclipse.xtext</groupId>
+            <artifactId>org.eclipse.xtext.xbase.lib</artifactId>
+            <version>2.37.0</version>
+        </dependency>
         <dependency>
             <groupId>org.springframework.boot</groupId>
             <artifactId>spring-boot-starter-test</artifactId>